An Interactive Approach to Formal Languages and Automata with JFLAP

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1 An Interactive Approach to Formal Languages and Automata with JFLAP Susan H. Rodger Duke University NSF CCLI Showcase March 9, 2007 Supported by NSF Grant DUE

2 Outline Overview of JFLAP Examples and Demo L-Systems Turing Machine Building Blocks Moore and Mealy Machines Pumping Lemma Batch Testing Mode JFLAP s use in Teaching JFLAP Study

3 Formal Languages and Automata Theory Traditionally taught Pencil and paper exercises No immediate feedback More mathematical than most CS courses Less hands-on than most CS courses

4 Why study finite automata? Application: Compiler Compiler identifies your syntax errors Can write a big DFA to identify all words in a Java program integers, doubles, boolean keywords, variable names arithmetic operators, punctuation symbols

5 Why Develop Tools for Automata? Textual Tabular Visual Interactive

6 Why Develop Tools for Automata? Examined 10 Automata textbooks One had software with book Only 6 had pictures of PDA, 2 or 3 states Only 6 had pictures of Turing machines, three of those switched representation Only 2 had picture of CFG to NPDA None had picture of parse tree for unrestricted grammar

7 Overview of JFLAP Java Formal Languages and Automata Package Instructional tool to learn concepts of Formal Languages and Automata Theory Topics: Regular Languages Context-Free Languages Recursively Enumerable Languages Lsystems

8 Thanks to Students - Worked on JFLAP and Automata Theory Tools NPDA , C++, Dan Caugherty FLAP , C++, Mark LoSacco, Greg Badros JFLAP , Java version Eric Gramond, Ted Hung, Magda and Octavian Procopiuc Pâté, JeLLRap, Lsys Anna Bilska, Jason Salemme, Lenore Ramm, Alex Karweit, Robyn Geer JFLAP , Thomas Finley, Ryan Cavalcante JFLAP Stephen Reading, Bart Bressler, Jinghui Lim

9 JFLAP Regular Languages Create DFA and NFA Moore and Mealy regular grammar regular expression Conversions NFA to DFA to minimal DFA NFA regular expression NFA regular grammar

10 JFLAP Regular languages (more) Simulate DFA and NFA Step with Closure or Step by State Fast Run Multiple Run Combine two DFA Compare Equivalence Brute Force Parser Pumping Lemma

11 JFLAP Context-free Languages Create Nondeterministic PDA Context-free grammar Pumping Lemma Transform PDA CFG CFG PDA (LL & SLR parser) CFG CNF CFG Parse table (LL and SLR) CFG Brute Force Parser

12 JFLAP Recursively Enumerable Languages Create Turing Machine (1-Tape) Turing Machine (multi-tape) Building Blocks Unrestricted grammar Parsing Unrestricted grammar with brute force parser

13 Finite Automata Editing and Simulation The most basic feature of JFLAP has always been the creation of automata, and simulation of input on automata. Here we demonstrate the creation and simulation on a simple NFA.

14 FA Edit & Simulation Start up JFLAP When we start up JFLAP we have a choice of structures. The first of these is the Finite Automata!

15 FA Edit & Simulation We start with an empty automaton editor window. Start Editing!

16 FA Edit & Simulation Create States We create some states...

17 FA Edit & Simulation Create Transitions We create some transitions...

18 FA Edit & Simulation Initial and Final State We set an initial and final state. Now we can simulate input on this automaton!

19 FA Edit & Simulation Input to Simulate... When we say we want to simulate input on this automaton, a dialog asks us for the input.

20 FA Edit & Simulation Start Simulation! When simulation starts, we have a configuration on the initial state with all input remaining to be processed.

21 FA Edit & Simulation After One Step This is a nondeterministic FA, and on this input we have multiple configurations after we Step.

22 FA Edit & Simulation The previous configurations on q 1 and q 2 are rejected, and are shown in red. The remaining uncolored configurations paths are not rejected, and are still open. After Two Steps

23 FA Edit & Simulation After Three Steps Yet another step.

24 FA Edit & Simulation After Four Steps One of the final configurations has been accepted!

25 FA Edit & Simulation Traceback One can then see a traceback to see the succession of configurations that led to the accepting configuration.

26 FA Multiple Run Select Multiple Run One can then enter many strings and receive acceptance info.

$biological systems and create fractals.$ Similar to Chomsky grammars, except

27 L-Systems may be used to model biological systems and create fractals. Similar to Chomsky grammars, except all variables are replaced in each derivation step, not just one! Commonly, strings from successive derivations are interpreted as strings of render commands and are displayed graphically. L-Systems

28 L-Systems This L-System renders as a tree that grows larger with each successive derivation step.

29 L-Systems L-systems may also be stochastic. The T Tg rule adds g to the derivation, which draws a line segment. We add another rewriting rule for T, T T. With two rewriting rules for T, the rule chosen is random, leading to uneven growth!

30 L-Systems The same stochastic L-system, rendered 3 different times all at the 9th derivation.

31 Students like L-systems

32 Turing Machine Building Blocks First, a problem. f(w) = number of a s in w, Examples: f(aabab) = 111 f(bbbaab) = 11 = {a,b}

33 Turing Machine Building Blocks Building Block Build a Turing machine with a specific purpose Name it and save it Use it as a BlackBox in another Turing machine Special Symbols ~ ignore read or write!x matches all symbols except for x

34 Simple Building Blocks start R move right R_blnk move right once, keep moving right until reach a blank

35 Simple Building Blocks (cont) R_not_a move right once, keep moving until not an a a - write a and stay put

36 Create & Combine Building Blocks New Buttons Load BB BB transition Conditional if the current symbol is b, move to the next block (tape head not moving) Move to the next block use ~ Ignore read, ignore write, stay put

37 Problem again: Count number of a s F(abbaabb) = 111

38 Building Block Run Choices

39 abbab Step by building block

40 abbab

41 abbab

42 abbab

43 abbab

44 abbab

45 abbab Skip a few steps to

46 Mealy Machines Similar to finite automata No final states Produce an output on their transitions deterministic

47 Example Vending Machine Dispenses candy once enough money has been inserted Money n(nickel), d(dime) q(quarter) Candy bars 20 cents Returns the appropriate amount of change the number of nickels C4 means candy and 4 nickels From Carroll and Long s Theory of Finite Automata book

48 Mealy Vending Machine Example

49 Moore Machine Similar to Mealy Machine No final state Output is produce by states, not transitions

50 Example Halve a Binary Number

51 Regular Pumping Lemma

52 Pick an Example

53 JFLAP Pump lemma Game User enters in steps 1 and 3

54 Context-Free Pumping Lemma

55 Similar CFL pump lemma game

56 CFL pump lemma (cont) Last step shows the contradiction In CFL there are lots of cases to consider

57 Batch Testing Mode Select several files for testing Then select input file

58 Batch Testing Mode (cont)

59 Using JFLAP in Teaching

60 Using JFLAP during Lecture Use JFLAP to build examples of automata or grammars Use JFLAP to demo proofs Load a JFLAP example and students work in pairs to determine what it does, or fix it if it is not correct.

61 Example: JFLAP during Lecture Ask students to write on paper an NPDA for palindromes of even length Build one of their solutions using JFLAP Shows students how to use JFLAP Run input strings on the NPDA Shows the nondeterminism

62 Example 2: JFLAP during Lecture Brute Force Parser Give a grammar with a lambdaproduction and unit production Run it in JFLAP, see how long it takes (LONG) Is aabbab in L? Transform the grammar to remove the lambda and unitproductions Run new grammar in JFLAP, runs much faster!

63 Parse Tree Results First Grammar 1863 nodes generated Second Grammar 40 nodes generated Parse tree is the same.

64 With JFLAP, Exploring Concepts too tedious for paper Load a Universal Turing Machine and run it See the exponential growth in an NFA or NPDA Convert an NPDA to a CFG Large grammar with useless rules Run both on the same input and compare Transform grammar (remove useless rules)

65 JFLAP s use Outside of Class Homework problems Turn in JFLAP files OR turn in on paper, check answers in JFLAP Recreate examples from class Work additional problems Receive immediate feedback

66 Ordering of Problems in Homework Order questions so they are incremental in the usage of JFLAP 1. Load a DFA. What is the language? Students only enter input strings. 2. Load a DFA that is not correct. What is wrong? Fix it. Students only modifying a small part. 3. Build a DFA for a specific language. Last, students build from scratch.

67 JFLAP Study Study of JFLAP s effectiveness in learning Runs Pretest/Posttest Interviews Supported by National Science Foundation, grant NSF DUE

68 Fourteen Participants Duke UNC-Chapel Hill Emory Winston-Salem State University United States Naval Academy Rensselaer Polytechnic Institute UC Davis Virginia State University Norfolk State University University of Houston Fayetteville State University University of Richmond San Jose State University Rochester Institute of Technology

69 JFLAP s Use Around the World JFLAP web page has over 110,000 hits since 1996 Google Search JFLAP appears on over 20,000 web pages Note: search only public web pages JFLAP been downloaded in over 160 countries

70 JFLAP is free! Questions? JFLAP book (Jones & Bartlett, 2006) Use as supplement to a textbook

An Interactive and Visual Approach to Learning Computer Science

An Interactive and Visual Approach to Learning Computer Science Susan H. Rodger Department of Computer Science Duke University November 30, 2004 his work was supported by: ational Science Foundation DUE-9752583